In recent years, energy consciousness has dictated much tighter construction of home and commercial dwelling units. As one aspect of this, there are now fewer internal air turnovers for any unit of time. In modern mobile homes as one example, when doors and windows are closed air turnovers typically range from 0.15 to 0.6 per hour, averaging about 0.3. While this tight construction has indeed helped to conserve energy it has spawned a host of new problems. Among these are those caused by internally generated moisture and odors. In this latter category, minute quantities of formaldehyde vapor generated from a multitude of internal sources can be cited as one example. Among the sources are tobacco smoking, natural gas cooking, carpeting, permanent press treatment of draperies and upholstering fabrics, and the urea-formaldehyde adhesives used in particleboard decking and in wall paneling. Formaldehyde evolution is usually more of a problem in hot, humid weather. The amounts emitted are very low and the ambient atmosphere of a modern mobile home may contain from 0.1 to 1.5 ppm formaldehyde vapor, or even more under extreme conditions. Typical values would be about 0.3- 0.7 ppm. Because of its highly irritating nature to eyes and the nasal mucosa, even these low amounts can cause discomfort to sensitive people.
To cure the problem it is necessary to control all of the sources of formaldehyde. Unfortunately, elimination of one or two sources does not guarantee that the remaining level of formaldehyde will be reduced in proportion to the original contribution of these sources. Effort must be made along multivariate lines of attack to completely resolve the formaldehyde problem. One part of this effort, as represented by the present invention, has been to better control formaldehyde emission from particleboard bonded by formaldehyde containing adhesives. Particleboard decking and cabinetry core products are widely used in mobile homes and are usually regarded as significant contributors to the internal formaldehyde level.
The art has long recognized that formaldehyde emission from composite products, such as particleboard bonded with urea-formaldehyde resins, can be in part controlled by the addition of free urea to the system. Urea acts as a formaldehyde scavenger, both at and subsequent to the time of manufacture of the product. The art is divided into three general ways of accomplishing this end. One way is to add the urea directly to the resin. Here it appears to tie up free formaldehyde that may be present in the resin. In effect, the molar ratio of formaldehyde to urea is decreased. This approach is exemplified in British Pat. No. 1,480,096. It is further discussed in British Pat. No. 2,019,854 and by A. A. Moslemi in Particleboard Volume I, Southern Illinois University Press, Carbondale (1974). This idea is attractive in its simplicity but it has not been particularly successful. Physical properties of the resultant product are severely affected when enough urea is added to gain significant reduction of formaldehyde evolution.
Another approach developed out of the failure of the one just described. This involved isolation of the urea from the resin in some manner so that it does not interfere with resin curing but is available later to scavenge formaldehyde. This often involves applying a solution of urea to the wood particles, for example as taught in British Pat. Nos. 2,019,854; or to a portion of the particles as in German Pat. No. 1,653,167; or to an inert carrier material, as in U.S. Pat. No. 3,983,084, in which a mixture of urea and an amylaceous material is added to the binder system. Again, the ideas are excellent in concept but in actuality none of these references cited seem able to reduce formaldehyde evolution to more than about 50% of the untreated level. Much better control has been a sought-after goal.
A third approach can be mentioned. This involves reformulation of the resins themselves, usually in the direction of lower molar ratios of formaldehyde to urea. Finally, other materials besides urea have been suggested, as in U.S. Pat. No. 4,186,242, in which ammonium lignosulfonate is used, or in U.S. Pat. No. 2,870,041 in which bisulfites are suggested for odor control in textiles. German DT 2847-975 shows the use of urea for formaldehyde absorption in pharmaceutical and cosmetic preparations. These approaches either stray away from practicality in the manufacture of wood products or, like the others, fail to give adequate control of formaldehyde evolution. Even more so they do fail to solve the problem for a secondary manufacturer who must purchase panels on the open market for remanufacture. Yet to the present time the secondary manufacturer has had little or no control over such aspects of his ultimate product as formaldehyde evolution, even though he would be likely to bear the brunt of any product liability claims.